1. Field of the Invention
The invention relates to a buffered acid composition and a method for manufacturing said buffered acid composition.
2. Description of Related Art
Buffered acids have many different utilities, including use as cleaning compositions as well as pH balancers. One important use of buffered acids is for the pickling of stainless steel. Pickling is the cleaning and removal from stainless steel of any type of high temperature-related scale caused by heat treatment or weld burn. Stainless steel that is heated by welding, heat treating, or any other means may develop a multi-colored oxide layer (referred to as “bluing”) on the surface. This oxidation is indicative of a chromium-depleted layer on the surface of the steel below the oxide layer. When the chromium content in stainless steel is decreased, the corrosion resistance of the stainless steel also decreases. Acid cleaners, also known as pickling agents, are the most effective means of restoring maximum corrosion resistance to oxidized stainless steel. Pickling is an acid treatment used to remove high temperature scale and red rust from the steel or from corrosion of contaminant iron and steel particles. High temperature dark scale is undesirable for aesthetic reasons and because the scale reduces the corrosion resistance of the underlying steel layer.
Several patents describe pickling agents produced from the mixture of an acid or acids and urea. U.S. Patent Application Publication No. 2003/0004080 describes a pickling agent containing urea and the method of producing that agent. In most pickling agents, nitric acid is used, however, when the nitric acid contained in those pickling agents oxidizes metal, harmful free radicals of nitrogen oxide (referred to as NOx) and nitrates are released as a by-product of the pickling process. Fumes of the various nitrous oxides are toxic to humans working in close proximity to the pickling agent, and both the fumes and the nitrates are environmental hazards. Alternative pickling methods utilize hydrogen peroxide, sulfuric acid, and Fe3+ as oxidizing agents, thereby circumventing the problems inherent in the usage of nitric acid. However, pickling compositions using these particular alternative agents are not as effective as compositions using nitric acid.
U.S. Pat. No. 5,672,279, issued to Sargent et al., on Sep. 30, 1997, describes a method for removing the accumulation of water-insoluble metal salts from surfaces using urea hydrochloride. The '279 invention does not use liquid ammonium combined with an acid to create that cleaning composition.
Nitrous acid (HNO2) is also used as in buffered acid compositions, however, nitrous acid is unstable and is always prepared in situ. Nitrous acid is usually made by reacting a solution containing a nitrogen source (such sodium or potassium nitrate (III)) with hydrochloric acid. The nitrous acid is a weak acid and undergoes the following reaction:H+(aq)+NO2−(aq)HNO2(aq)Because nitrous acid is a weak acid, the position of equilibrium lies well to the right of the equation. Furthermore, the reactant solution will contain various ammonium ions and nitrite ions and also chloride ions from the hydrochloric acid. These ions remain in solution and are undesirable due to their toxicity.
A buffered acid solution is needed in the art which does not contain nitric acid or nitrous acid and which does not produce toxic flues or free radicals that are harmful to humans or to the environment, but that is effective for purposes of cleaning water-insoluble mineral and chemical accretions from hard surfaces as well as for balancing the pH of substances when needed.